In subterranean formations which are regular geotechnical non-seismic formations, either soft ground or hard, there is little or no risk of compressive or deformation stresses between the subterranean formation and any subterranean structure in the subterranean formation and thus little or no risk of potential damage of the subterranean structure. However, there are many situations where subterranean structures are placed under compressive or deformation stresses. One example is where a structure is located in a seismic zone subject to earthquakes. In those situations, the compressive and deformation stresses from an earthquake are transferred through the subterranean formation and can cause cracking and damage to subterranean structures such as foundations of buildings, underground tunnels or utility service mains or piping.
Another situation is if the subterranean structures are located in time dependent rock formations (swelling rock), where the deformations can be significant and forces from the rock could damage the subterranean structure over a period of time. This is particularly true in the case of bored tunnels.
Bored tunnels using a Tunnel Boring Machine (TBM) are typically lined with a permanent concrete lining material. Concrete liners can be either cast in place or can be precast segments joined together with the gaps between the segments being filled with a compressible gasket. The selection of the lining material is generally based on the tunnel function and the nature of the rock or soil through which the tunnel is bored.
In time dependent rock formations (swelling rock), the deformations are significant and forces from the rock could damage the concrete liner over a period of time. In such cases, there is a time dependent deformation (TDD) of the rock that can, if not accommodated by the tunnel lining, result in a reduction of the tunnel diameter and potential damage to the tunnel liner. TDD is movement that occurs after the initial ground disturbance that results from tunneling. There are two distinct mechanisms that result in TDD, swelling and squeezing, although they may occur simultaneously and one may lead to the other. Swelling is the time dependent volume increase of the ground and squeezing is the time dependent shearing of the ground. Both phenomena lead to inward movement of the tunnel perimeter. TDD generally occurs over a period of about 3 months to a few years after which time the pressures and forces are balanced and thus relieved.
In the past, in such time dependent deformation rock formations, typically the tunnel liners were cast in place liners constructed after 3 months. In such situations, a temporary liner would be installed. This could result in significantly increased costs of the tunnel construction as a result of the delay in installation of the final liner.
It has been proposed in such situations to use precast tunnel liner segments where the tunnel is bored to a larger diameter than the finished diameter and to utilize a compressible mortar in the gap between the tunnel liner and the tunnel wall. One such compressible mortar is described in EP Patent No. 1,790,624 issued Nov. 28, 2006, to Hochtief Construction AG. This is also described by B. Billig et. al., Deco Grout—Innovative Grout to cope with rock deformations in TBM tunneling, in Underground Space—the 4th Dimension of Metropolises, Bartak et. al., eds. (2007), pages 1487-1492. As described in these references, the grout utilizes expanded polystyrene beads along with foam to achieve a grout being capable of being compressed to about 50% of its initial volume. In the studies, they tested expanded clays but rejected them as not being suitable. Also, they developed the grout independent of the actual forces or pressures as they state that these can only be calculated in advance with great difficulty and thus they attempted to develop a grout with a wide envelop of compressibility.
There are unique situations where, in addition to the TDD of the rock, if there are fluids present in the rock, the fluids may be released as water or mud, depending on the rock structure. In these situations, the presence of the fluid may impact the curing of the compressible mortar used as a fill and the compressible mortar may either be washed out or any compressible material in the mortar may float to the surface and not be maintained universally distributed throughout the mortar.
There thus remains a need for a compressible grout mix which can be formulated to provide the optimum compressibility particularly for the specific application where fluid may also be present.